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I was surprised when I looked at the power rating of the engine used on a Humvee. It's only ~190 horsepower, which is exceeded by many sedan engines.

So an obvious question is why doesn't my Camry SE burn more gas than a Humvee and I think it's just because during regular use, it doesn't use anywhere near 250 HP.

Does anyone know about how much mechanical power a typical sedan engine puts out during highway driving?

Also, can someone speculate what the torque vs RPM graphs of a sedan engine and a Humvee engine look like and what the typical operating points are?

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A lot of the fuel consumption issue is just about the mass of the vehicle, and another big chuck is about drag coefficients. Imaging the extra cost of putting five hundred pounds of extra steel into the frame of the sedan. A thousand. More. Then put a sail on top. –  dmckee Mar 12 '13 at 19:01
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2 Answers 2

up vote 3 down vote accepted

Find a long straight hill, take your foot off the gas and see what terminal speed you reach. From the rate of descent and the slope you can work out the power (in this case potential energy) required to maintain that speed.

For my typical modern aerodynamic small car I think it worked out at around 30-35HP to maintain typical highway (100kph) speeds.

edit: manual transmission and modern engine management system, so I'm assuming it wasn't actually burning any fuel

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OK, I see averageEnergyLoss = (m g h - 0.5 m v^2) / dt. That's a clever way to measure - I'll try it some times. –  Yale Zhang Mar 12 '13 at 19:51
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-exactly, I was bored driving through Utah, they have long straight hills with the grade (slope in deg) posted –  Martin Beckett Mar 12 '13 at 20:02
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I thought of another empirical estimate based on the heat released from gasoline. I use 15.6 gallons between refueling, so that's 521 kW*h / (10 hours) = 70 HP. So 35 HP sounds about right if the efficiency is 50%. –  Yale Zhang Mar 12 '13 at 20:06
    
And if your gearbox is well designed this means that the engine is just purring along at about fast idle speeds or a little less (say 1000-1200 RPM for typical sedans). –  dmckee Mar 12 '13 at 20:18
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You might be interested in my answer to If an electric car were to drive without having to stop, would the range be greatly affected by the speed at which the vehicle is moving?. From just air resistance I calculated that at 70 mph the engine in my Ford Focus is generating about 17 bhp (12.5 kW).

Obviously you need to add in losses in the transmission. Apart from anything else the above result predicts a top speed of about 150 mph and I can assure you my Focus isn't that fast! This graph suggests that the rolling resistance is about 0.8 of the aerodynamic resistance at 70 mph, and that would make the total power about 30 bhp - reassuringly close to Martin's estimate.

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